The equipment for flexible AC transmission system (FACTS) can be divided into series compensation devices, parallel compensation devices and integrated control devices. The parallel compensation devices can be directly connected to various levels of power grids, while series compensation devices and integrated control devices need to be connected to power grids in series at one end, and thereby their connection modes should be studied in comprehensive consideration of reliability, flexibility and security thereof.
In series compensation devices and integrated control devices, static synchronous series compensator (SSSC), unified power flow controller (UPFC), inter-line power flow controller (IPFC) and convertible static compensator (CSC) are all flexible power transmission devices that can improve the transmission and control capabilities of the system; there is also a unified power quality regulator (UPQC), which can improve the power quality of lines. In addition to the static synchronous series compensator, other device includes two converters and the corresponding transformer to perform isolation and transformer functions. Since the static synchronous series compensator is generally performed as an additional operation mode for other devices, it can also be included in the same kind.
At present, the series end of series compensation devices or integrated control devices is connected to power grids through series transformers. The 110 kV and above power grids in China mostly adopt double-circuit line structure, which requires two series transformers to connect two sets of series compensation devices to double-circuit lines respectively, which inevitably increases area occupation and investment costs, and at the same time, multiple series transformers also increase the overall loss. For power grids with more circuit structures, cost, area occupation and equipment loss will further limit the application of series compensation devices. In order to solve the above shortcomings and improve the economy of FACTS access to power grid, a series compensation device more suitable for double-circuit lines is needed.